Bibliographic details of the publications referred to by number in this specification are collected at the end of the description.
The development of small therapeutic agents is a major goal in the pharmaceutical industry. Such agents are potentially relatively inexpensive to manufacture and are less likely to induce adverse immunological responses. One of the difficulties, however, in small therapeutic molecule development is target selection. Many potential targets lack suitability due to their pleiotropic nature and/or due to the level of redundancy in a particular pathway.
p53 (also known as protein 53 or tumour protein 53), is a transcription factor which in humans is encoded by the TP53 gene. p53 is important in multicellular organisms, where it regulates the cell cycle and functions as a tumor suppressor that is involved in the prevention of cancer. As such, p53 has been described as “the guardian of the genome,” “the guardian angel gene,” and the “master watchman”, referring to its role in conserving stability by preventing genome mutation.
p53 mutation can corrupt the tumor suppressive functions of the wild type (wt) protein. In response to specific cellular stresses, wt p53 either initiates a temporary interruption of the cell cycle to enable DNA repair or triggers cellular senescence or apoptosis when damage is excessive (reviewed in Meulmeester et al.). In contrast, cells bearing common p53 mutations are released from these constraints. Further, certain p53 mutations may acquire distinct properties from the wild type counterpart, referred to as a “gain of function” (GOF) phenomena (reviewed in Strano et al. (Head Neck)). The contribution of certain p53 mutations to chemotherapeutic drug resistance was demonstrated in human cancer cells cultured in vitro (reviewed in Sigal et al. and Strano et al. (Oncogene)). A role for mutant p53 in the development and spread of the tumors was also corroborated by these studies. The most compelling evidence for the GOF of mutant p53 was demonstrated in knock-in (KI) mutant p53 mice, which developed tumors with a distinct spectrum from p53+/− or −/− mice. Furthermore, tumors in these KI mice exhibited an enhanced metastatic potential (Lang et al. and Olive et al.)
Wild-type p53 is subject to tight regulation that is affected through protein-protein interactions and by extensive post-translational modifications (Lavin et al. and Bode et al.). In contrast, less is known about the regulation of mutant p53, although mutant p53 is subject to at least certain modifications (Bode et al.). Strikingly, higher mutant p53 levels have been identified in (most) tumor cells than in the surrounding healthy tissues of mutant p53 KI mice (Iwakuma et al.). Recent work by Terzian et al. demonstrated that, as in the case of wt p53, the stabilization of mutant p53 is regulated by Mdm2. Mice lacking mdm2 express higher levels of mutant p53 succumb earlier and develop metastatic tumours (Terzian et al. and Prives et al.).
Certain modifications of wt p53 are regulated by the promyelocytic leukemia (PML) protein (Zimber et al.). PML is a key factor in the formation of PML nuclear bodies, which are distinct nuclear multi-protein complexes that have been associated with critical cellular processes, including tumor suppression, gene regulation, post-translational modifications and protein catabolism (reviewed in Zimber et al. and Salomoni et al.). PML knock-out mice develop normally, but are resistant to lethal doses of γ-irradiation (IR; Guo et al.). In addition, they are prone to tumorigenesis in response to carcinogens (Wang et al.), or an additional oncogenic event, such as the loss of PTEN (Trotman et al.). In humans, a complete or partial loss of PML has been observed in multiple types of cancers, including breast colon and prostate (Gurrieri et al.). PML is therefore considered as a bona-fide tumour suppressor.
WO 2009/063426 discloses siRNA mediated knock-down of the PML/RARα (retonoic acid receptor alpha) complex which inhibits the growth of Acute Promyelocytic Leukemia (APL) cells. However, it is the chromosomal translocation t(15; 17) of the PML gene which causes the formation of an aberrant fusion protein between PML and RARα. RARα then targets the complex to myeloid specific genes with consequent inhibition of myeloid cell differentiation. Thus, the siRNA is used to target a mutant form of PML specifically for the treatment of Acute Myeloid Leukemia (AML).
US 2003/0207791 discloses methods for the treatment of a disease associated with the formation of high molecular weight complexes of chimeric transcription factors. This includes PML/RARα and its use in the treatment of acute myeloid leukemias. Again, the agent is used to target a specific transcription factor complex for the treatment of AML.
The regulation of mutant p53 and the cellular events leading to tumour cell growth and associated hyperproliferative disease is not well understood. Identifying possible cellular targets which affect the growth and/or survival of tumour cells characterized by mutant p53 expression may lead to the identification and development of effective cancer therapies.